1. Field of the Invention
The present invention relates to semi-conductor processing equipment, more specifically to an improved target assembly for ameliorating the effect of back sputtering during vapor deposition.
2. Background of Related Art
In semiconductor processing, and more specifically in physical vapor deposition, a target is disposed on a backing plate in the upper portion of a chamber and a plasma is struck between a work piece and the target to produce ions which bombard the target and result in the deposition of target material onto the work piece. While most processes are finely tuned to result in deposition of target material onto the work piece in a trajectory which is normal to the work piece, some particles are redeposited on or near the target itself. These particles become a source of contamination which loosely adhere to the target and surrounding areas of the chamber and may eventually flake off and fall onto the work piece.
Efforts aimed at reducing the concentration of particles in sputter chambers have taken many different approaches. One approach is to periodically clean the chamber using a wet-clean process. Another approach is to clean the chamber with a cleaning gas that reacts with the deposits. The gaseous by-products of these reactions are exhausted from the chamber. However, in both of these approaches, the deposition process must be discontinued during the cleaning process and additional time must be taken for the cleaning materials and by-products to be evacuated from the chamber before the deposition process can resume.
Another method for managing the particle concentration in a chamber involves the use of sputter shields that prevent sputtered particles from depositing directly on the chamber walls. The sputter shield is periodically replaced as part of the process kit so that buildup of potentially harmful deposits never gets out of control. This method can reduce the frequency at which the chamber must be cleaned, but a fraction of the particles still pass around the shield and eventually form deposits that must be cleaned.
Another approach is to replace what is known as the processing kit components. Typically, a processing kit includes both a cylindrical sputter shield and a clamp ring which holds the work piece on the support member. The cylindrical shield is suspended in the chamber below the target and prevents deposition on the chamber walls, while the clamp ring is disposed in the chamber near the work piece to expose one surface of the work piece to the target. The cylindrical shield and clamp ring are consumables that are periodically removed from the chamber and replaced with new components. However, the target is an expensive item and is not replaced as often as the shield and clamp ring. Therefore, the target still accumulates particles that can result in product failure and frequently must be replaced prior to the planned life of the target.
The problem of redeposition of sputtered material back onto the target sidewall has also been recognized as an undesirable source of particles in the chamber. Sputtered particles that become scattered in the chamber atmosphere can redeposit onto the side of the target and accumulate to form particles of the deposition material. Because RF or DC power is applied to the target during sputter deposition on a substrate and then removed from the target between substrates, the target, as well as the redeposited material, is continually heated and cooled and thereby subjected to thermal stress. Over a period of time, this stress will cause particles of the redeposited material to come loose and fall onto the substrate.
U.S. Pat. No. 5,334,298 describes a sputtering cathode which was designed to reduce sputtering around the edges of a target. A "dark space shield" was added to expose only one surface of the target and stop ignition of plasma around the perimeter of the target. Such a shield could be replaced along with other components as needed to reduce particle contaminants.
The target in a physical vapor deposition system is typically attached to a baseplate which serves as a lid to the deposition chamber. The portion of the baseplate surrounding the outer perimeter of the target is an area in which back sputtered particles tend to deposit, and then, over time, fall to deposit on the substrate.
Therefore, there exists a need to provide a method and apparatus which provides an easy and cost effective way of removing particles from the sputter chamber. More particularly, there is a need for a method and apparatus for periodically cleaning the surfaces around a target in a process chamber on which back sputtered particles may deposit. It would be most desirable if the apparatus and method could be carried out in accordance with usual and customary cleaning processes in order to maintain the substrate throughput of the system.